download pdf - Utah Geological Survey - Utah.gov

More documents

Recommendations

Info

6 from fine grained to pebbly. Where fresh, the mudstone breaks conchoidally into small to large, angular, equidimensional fragments. The Ruby Ranch Member generally forms "bumpy" to smooth-weathering, light-gray to light-greenish-gray slopes with abundant carbonate nodules littering the slopes. Where the Buckhorn Conglomerate Member is not present, light-hued, medium to thick mudstone beds that vary in resistance and steepness form a continuous slope from the top of the Salt Wash Member to the base of the Mussentuchit Member. The Brushy Basin Member slopes are generally smoother than the Ruby Ranch Member mudstones. Subtle, slightly less variegated, and duller overall color banding, indicative of variations in the mudstone bedding, is more common in the Ruby Ranch Member. The age of the Ruby Ranch Member is Early Cretaceous (Aptian-Albian) according to fossil evidence and radiometric and stratigraphic relations (Kirkland and others, 1999; Kirkland and Madsen, 2007). The Ruby Ranch Member is generally present everywhere the Cedar Mountain Formation is exposed throughout the Colorado Plateau region, but is missing in southwestern Utah (Kirkland and others, 1999; Jim Kirkland, Utah Geological Survey, verbal communication, 2008). Short Canyon member Short Canyon member: We introduce a new informal member of the Cedar Mountain Formation with a type section located on the north side of Short Canyon in section 20, T. 21 S., R. 8 W. The member is discontinuous and most prominent in the north half of the quadrangle. It represents a channel deposit that preceded and was partly contemporaneous with the deposition of the Mussentuchit Member and is much like the Buckhorn Member at the base of the Ruby Ranch Member. In the north half of the Short Canyon quadrangle it resembles the Dakota Formation as well. The Short Canyon member consists of one to three resistant ledges of conglomerate, conglomeratic sandstone, and sandstone, that are separated by slope-forming sandstone, local gray to black carbonaceous (and possibly coaly) shale, and rare, thin gypsiferous sandstone lenses. The ledges and cliffs are various shades of gray, gray brown, and brown, and are cross-stratified and commonly trough cross-bedded. The conglomerate contains poorly sorted, subrounded to subangular grit to cobbles as much as 4 inches (10 cm) across. The clasts consist of quartzite, siliceous limestone, and sandstone and are poorly to well cemented with calcite or silica. The abundance of quartzite clasts distinguishes the Short Canyon member from the Buckhorn Conglomerate (Gary Hunt, Utah Geological Survey, verbal communication, 2009). The sandstone in the ledges is also poorly sorted, and some beds are very fine to medium grained but others are fine to coarse grained. The cementation varies in degree, and is calcareous or siliceous, but the rock is mostly firm. The ledges probably Utah Geological Survey represent channel deposits of an ancient river system. Carbonaceous shale and coaly shale as much as 2 feet (0.6 m) thick and rare gypsiferous sandstone lenses are present in the highest parts of the principal ledges. Slope-forming, muddy sandstone separates the ledgeforming rock. Beds are fine to medium grained; light gray, green gray, orange, and light brown; poorly cemented; and friable. Cementation is principally calcareous. Locally present in the earthy-weathering slopes are very small rip-up clasts of nearly white mudstone, and well-indurated sandstone forms a slight ledge in the slope. The slope-forming, muddy sandstone is generally lighter and less structured than mudstone in the Mussentuchit Member. As ascertained by Kirkland and others (1999), the Ruby Ranch and Mussentuchit Members contact is an unconformity. The Short Canyon member is a channel-form deposit laid down above the plane of unconformity. Such channels are not present in the southern part of the quadrangle, but subparallel the Cedar Mountain Formation outcrop belt in the north. The Short Canyon member continues in outcrop into the Molen quadrangle to the north. A channel or set of channels is thickly developed where the Cedar Mountain Formation outcrop belt crosses Short Canyon. There are at least three channel ledges: the lowest cuts deeply into the Ruby Ranch Member, and the highest rises into the Mussentuchit Member above. Although we could not actually pinpoint a place, the Dakota Formation may locally rest directly on the Short Canyon member. However, at one location we measured 104 feet (32 m) of the Short Canyon member and only 38 feet (12 m) of the Mussentuchit Member. The lowest ledge is widest and the highest is the narrowest. The lower contact with the Ruby Ranch Member is wavy and irregular. The upper contact with the Mussentuchit Member is relatively abrupt and discordant to regional bedding and probably represents the boundary between the sides of channels and overbank or lacustrine deposits. Mussentuchit Member Mussentuchit Member: The Mussentuchit Member unconformably overlies the Ruby Ranch Member wherever they are in contact. Where it overlies the Short Canyon member, the contact is conformable. The Mussentuchit Member is continuous across the quadrangle ranging from 38 to 110 feet (12–33 m) thick, and generally thins northward. It differs from the Ruby Ranch by the lack of nodular limestone horizons and by an increase of volcanic ash material in the mudstone. The type section of this member was designated by Kirkland and others (1997) at a location on the south side of Mussentuchit Wash in the Willow Springs quadrangle, about 12 miles (19 km) to the south. The upper contact with the Dakota Formation is an unconformity. The mudstone of the Mussentuchit Member weathers into light-gray, white, and light-green-gray colors. The mudstone is smectitic, sandy, or silty. Ledges locally include
Geologic map of the Short Canyon quadrangle, Emery County, Utah 7 some medium green-gray, fine- to medium-grained sandstone. Like the Ruby Ranch, differences in resistance afford variable slopes with changes in slope angle. Locally, some dark lignitic horizons are present, especially near the top of the unit. The radiometric age of the Mussentuchit Member has been established as 98.39 Ma obtained by Cifelli and others (1997), and Garrison and others (2007). Thus, this member is considered Cenomanian in age based on Gradstein and Ogg (2009). The Mussentuchit Member has also yielded a rich dinosaur fauna. The member crops out mostly along the west flank of the San Rafael Swell and extends into southwest Utah (Biek and others, 2003; Biek and others, 2007). It correlates to the Mowry Shale in northern Utah (Sprinkel, 2007) but is not present in many parts of the Colorado Plateau region. Dakota Formation The Dakota Formation is also locally known as the Dakota Sandstone. Inasmuch as the Dakota is mostly shale in the Short Canyon quadrangle, we use the term “formation.” In its full development in the quadrangle, it consists of a lower slope, a lower sandstone ledge or cliff, a middle slope, an upper sandstone ledge or cliff, and a thin, pebbly upper slope containing Pycnodonte newberryi (oyster shell fossils). The five divisions are not developed everywhere in the quadrangle; in many areas the upper sandstone ledge is thin or missing, and in other areas the lower sandstone ledge is thin or missing. The lower boundary is an unconformity. The upper boundary is mapped where overlying deeper marine (Mancos Shale) deposits first occur. In some areas of the San Rafael Swell, a local unconformity has been identified near the base of the upper slope that contains pebbles and Pycnodonte newberryi fossils (Eaton and others, 1990; Jim Kirkland, Utah Geological Survey, verbal communication, 2008). However, for mapping purposes we have included the thin slope interval in the Dakota Formation following Lawyer (1972) and Peterson and others (1980). The Dakota is 50 to 100 feet (15– 30 m) thick. The thickness varies, but is generally thickest in northern exposures. The Dakota Formation consists of interbedded sandstone, shale, carbonaceous shale, coal, conglomeratic sandstone, and conglomerate. The sandstone ledges or cliffs consist of thin to thick lenses of sandstone and conglomerate. The sandstone beds are generally light hued, including yellow, orange gray, light tan gray, or very light brown. They are commonly fine to medium grained, but locally they contain rounded or subrounded pebbles and cobbles that grade into gray conglomerate; most are well to moderately sorted. Partings in the ledges and cliffs consist of carbonaceous shale or gray shale. The conglomerate and sandstone commonly exhibit trough cross-beds. Burrows and plant imprints are locally common. The carbonaceous shale partings commonly contain plant debris. The rock is moderately cemented with calcite. Locally, the sandstone contains pyrite nodules, but most have altered to brown limonite. The slopes contain irregularly bedded mudstone, siltstone, sandy shale, gray shale, carbonaceous shale, coal, muddy sandstone, and brown sandstone. Except for the brown sandstone, the strata weather into slopes and recesses. The most common colors are shades of gray or gray brown. Coal beds, where present, are very thin and lenticular, rarely exceeding 6 inches (15 cm) in thickness, although one may be up to 1.5 feet (0.5 m) thick. Except for the brown variety, most sandstone beds are friable and fine grained. The brown variety of sandstone is typically thin-bedded and cemented with calcite. The Dakota Formation is Late Cretaceous (Cenomanian) in the Short Canyon quadrangle (Eaton and others, 1990; Molenaar and Cobban, 1991). Mancos Shale The Mancos Shale is represented by three members on the west side of the Short Canyon quadrangle. These are (ascending order) the Tununk Member, Ferron Sandstone Member, and Blue Gate Member. The Mancos Shale was deposited during the time of the Cretaceous Western Interior Seaway. Tununk Member Tununk Member: The Tununk is the oldest of the Mancos Shale members and represents the westernmost incursion of the Western Interior Seaway. In the quadrangle, it is present beneath the Molen Reef in a southwest-tonortheast band as a steep concave slope; locally as a vertical cliff. We measured the Tununk Member in the NW 1/4 section 14, T. 22 S., R. 7 E., where it was about 675 feet (205 m) thick. The Tununk Member is composed of marine mudstone and shale that is medium to dark brown or gray and locally black, and that breaks into equidimensional fragments when excavated. The unit is soft, silty, and sandy (fine grained) near the bottom and top. Bedding is mostly indistinct to thin and laminated. The Tununk contains secondary gypsum veinlets and is mostly calcareous. In the upper 50 feet (15 m), the shale is interbedded with thin, brown to gray, very fine grained sandstone beds that are about 1 foot (0.3 m) apart. The upper contact is mapped at the base of the first medium or thick sandstone bed of the overlying Ferron Sandstone Member. About 100 feet (30 m) below the top of the member is a conspicuous ledge of sandstone, about 25 feet (8 m) thick, which we informally call the Moore sandstone bed (not mapped). The sandstone is light-gray, fine grained, blocky, and massive.
Page 1 and 2: GEOLOGIC MAP OF THE SHORT CANYON QU
Page 3 and 4: STATE OF UTAH Gary R. Herbert, Gove
Page 5 and 6: GEOLOGIC MAP OF THE SHORT CANYON QU
Page 7 and 8: Geologic map of the Short Canyon qu
Page 9: Geologic map of the Short Canyon qu
Page 19: Utah Geological Survey a division o

download pdf - Utah Geological Survey - Utah.gov

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?